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Along with cooking a turkey and buying presents for the kids, it’s time to add getting a COVID-19 booster shot to your holiday to-do list.

The Food and Drug Administration on Friday granted emergency use authorization for a third dose of Pfizer and Moderna vaccines to all fully vaccinated adults.

Of the 194 million fully vaccinated adults in the country, more than 21 million have already received a booster shot. Boosters became available to people 65 and older or to people who have an underlying health condition in September.

The Centers for Disease Control and Prevention are expected to issue more specific guidance on boosters following the FDA’s move. In the meantime, here are the answers to some common questions about COVID-19 booster shots:

What is a booster?

Boosters do just as their name implies — boost immunity. Scientists have found immunity to COVID-19 within vaccinated individuals can start to wane around six to 12 months following a second dose of the Pfizer or Moderna vaccine, especially for individuals 65 or older or people who are immunocompromised, said Dr. Duane Hospenthal, director of infection control for the Baptist Health System. A booster can strengthen people’s immunity again, making it less likely that they could get sick and hospitalized.

“The way we look at vaccinology is that you get the immune system’s attention with those first two shots, and then, later on, you get a reminder shot to really apply the long-term memory,” Hospenthal said. “The hope is that by doing these boosters, that you’re going to get years of protection.”

Why is a booster necessary for COVID-19?

While some vaccines give the recipient long-lasting immunity, such as the measles vaccine, others need boosters. Boosters are commonly needed for respiratory viruses, like the flu, which can quickly evolve, said Dr. Jason Bowling, University Health epidemiologist.

For example, within COVID-19, the delta variant is much more transmissible than the original alpha strain. This variant of COVID-19 further increases the need for boosters, Bowling said.

“It’s this waning immunity, also in the context of a more infectious strain, that led to this desire to boost people’s immune system up again — so we have more protection, so we have less spread of disease,” Bowling said.

Bowling stressed that the need for a booster does not mean the COVID-19 vaccine doesn’t work. It’s still the best method of protection we have against the virus, he said.

A recent survey from the Texas Department of State Health Services underscores how well vaccines work. It found that of the nearly 29,000 Texans who’ve died from COVID-related illnesses since January, only 8% were fully vaccinated.

Bernard Arulanandam, microbiologist and UTSA’s vice president for research, economic development and knowledge enterprise, said boosters are quite common.

When it comes to battling viruses, it’s all about making sure the body’s cells remember to fight foreign agents like COVID-19, but sometimes they need a little reminder, Arulanandam said. Boosters work by extending the length of the body’s protective responses, he added.

What’s the difference between a booster and a third dose?

Immunocompromised people who received one of the mRNA vaccine series (Pfizer or Moderna) require a third dose. Moderna’s booster shot is half the size of its third dose, while the third dose size is the same as the booster for the Pfizer and Johnson & Johnson vaccines. However, the third dose is not the same as the booster, Bowling said. Immunocompromised people can get a booster six months after receiving a third dose of the vaccine.

“There’s a third dose for people that have moderate to severe immune system issues,” Bowling said, acknowledging that “adds to the confusion around the booster.”

Chemotherapy patients, organ transplant recipients, and people with chronic illnesses are among those who need a third dose, Bowling said. See a full list of third-dose qualifications here.

The booster is for “everyone else” who is currently eligible, Bowling said. As of Friday, that is now all fully vaccinated adults 18 or older. Bowling highly recommended people who are 65 or older, who have underlying health conditions, and/or who are frontline workers get the booster.

People who get the third dose will be eligible for a booster six months after receiving it, said Dr. Anita Kurian, assistant director of Metro Health. Anyone unsure of whether they need a third dose versus a booster should consult their doctor, Kurian added.

Who needs a booster?

Currently, those 18 or older who received one of the mRNA vaccine series (Pfizer or Moderna) six to 12 months ago are eligible. Recipients of the one-dose Johnson & Johnson vaccine are also eligible for a booster if they received their shot two or more months ago.

But some populations face more risk than others.

“The group that [doctors are] most interested in for boosters are people that are 65 and older,” Bowling said. “Because similar to the flu, these are the people that are most likely to be hospitalized, end up in ICUs, or die from COVID-19.”

Bowling also recommended everyone who is a frontline worker — such as hospital employees, first responders, teachers, service care workers and retail/hospitality workers — get a booster as soon as they can, since they come face to face with more people on an average day.

That should definitely include nursing home residents and employees, Arulanandam stressed, noting that in COVID-19’s early days the virus ravaged these communities because many residents have weak immune systems.

“That definition of frontline is so broad,” Arulanandam said. “I think those really are the folks that that really should be first in line to get the boosters and then … the general population.”

What are the potential side effects from a booster?

Similar to the first and second doses, recipients of the third dose or booster may have some side effects from another jab, Kurian said. These side effects can include soreness at the injection site, headaches and fever. See a full list of COVID-19 vaccine side effects here.

Kurian recommended shot recipients try to get their booster on a Friday or weekend day if they are concerned about needing more recovery time. Residents should prioritize getting their second dose in the series if they have only received one of the shots for one of the two-dose series (Pfizer and Moderna). As of Thursday, 77.4% of San Antonians have received at least one of their shots and 64.4% are fully vaccinated.

What about kids?

At this time, children are not eligible for boosters, Kurian said. Boosters are only available to adults 18 and older who are already fully vaccinated.

To read more about children’s COVID-19 vaccinations, check out our recent Q&A here.

Where can I get a booster?

Bexar County residents can get a free booster shot or third dose at the Alamodome Wednesdays through Fridays from noon to 8 p.m., or via one of the City’s pop-up health clinics. No appointment is necessary, according to the city’s website. While paperwork is not required, Kurian recommended recipients bring their vaccination card. If they have lost it or cannot find it, they can be looked up in the vaccine registry, she said. For any additional questions, call 311 or 210.207.6000  and select option 8, or email

Most pharmacy retailers, such as Walgreens, CVS, H-E-B, and Walmart also are offering boosters at no cost, and many are offering a free flu shot at the same time. It is safe to get both in one sitting, Kurian said. An appointment may be required.

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Lindsey Carnett

Lindsey Carnett covers the environment, science and utilities for the San Antonio Report.